1. Field of the Invention
The present invention relates to an imaging optical system and an image reading apparatus equipped with the same. The present invention can be suitably applied to image scanners and digital copying machines that can read images at high resolution with well-balanced correction of various aberrations in reading monochrome images and color images using a line sensor.
2. Related Background Art
Heretofore, there have been proposed various image reading apparatuses for reading image information on a original using a line sensor (CCD) having a plurality of light receiving elements arranged along the main scanning direction in which image information on the original or the like is read using an output signal obtained from the line sensor while the relative position of the original and the line sensor is changed along the sub-scanning direction.
FIG. 13 schematically shows the basic structure of a conventional image reading apparatus using a carriage-integrated scanning system. In the arrangement shown in FIG. 13, a light beam emitted from an illuminating light source 1 directly illuminates an original 7 placed on an original table glass 2. The optical path of a reflected light beam from the original 7 is deflected in the carriage by first, second and third turn back mirrors 3a, 3b and 3c, and then the reflected light is focused on a surface of a line sensor 5 by an imaging lens (imaging optical system) 4.
Image information of the original 7 is read by moving the carriage 6 in the direction indicated by arrow A in FIG. 13 (the sub-scanning direction) by a sub-scanning motor 8. The line sensor 5 shown in FIG. 13 is composed of a plurality of light receiving elements aligned along one dimensional direction (the main scanning direction).
FIG. 14 illustrates the basic structure of the image reading optical system shown in FIG. 13.
In FIG. 14, reference sign 144 designates the imaging optical system. Reference signs 145R, 145G and 145B respectively designate line sensors for reading image information of respective colors R (red), G (green) and B (blue), the line sensors 145R, 145G and 145B constituting the line sensor 145. Reference signs 147R, 147G and 147B designate reading areas on the surface of the original 147 corresponding to the line sensors 145R, 145G and 145B, respectively.
By scanning the surface of the original 147, it is possible to read the same point with different colors at a certain time interval. In the case where the imaging optical system 144 in the above-described structure is an ordinary refracting system, longitudinal chromatic aberration and chromatic aberration of magnification occur, which leads to defocus and misregistration in line images imaged on the line sensors 145B and 145R relative to the reference line sensor 145G. Consequently, an image reproduced by superimposing the images of the respective colors will suffer from noticeable color blurring and misregistration. Therefore, this apparatus cannot meet demands for large aperture and high resolution.
Meantime, in connection with decentered optical systems, it has recently been disclosed that it is possible to construct an optical system in which aberrations are satisfactorily corrected by introducing a concept called a reference axis and designing a constituent surface as an asymmetrical aspherical surface (see Japanese Patent Application Laid-Open No. H09-005650, U.S. Pat. No. 5,825,560 and U.S. Pat. No. 5,847,887). Japanese Patent Application Laid-Open No. H09-005650 discloses a method of designing such systems, and U.S. Pat. No. 5,825,560 and U.S. Pat. No. 5,847,887 disclose some designs of such systems.
Such a decentered optical system is referred to as an off-axial optical system. (The off-axial optical system is defined as an optical system that includes a curved constituent surface (off-axial curved surface) whose surface normal at the point of intersection with the reference axis (which is an axis defined along a beam that passes through the center of the image and the center of a pupil) is not on the reference axis, the reference axis of the off-axial optical system being a polygonal line.)
In off-axial optical systems, constituent surfaces are generally decentered, and vignetting does not occur even in reflecting surfaces. Accordingly, it is easy to construct an off-axial optical system using reflecting surfaces. In addition, by the nature of the off-axial optical system, routing of the optical path can be designed relatively freely, and it is easy to produce an integrated optical system by integrally molding constituent surfaces.
There have been disclosed imaging optical systems used for image reading to which the above-described technology is applied (see US AA2003038228 and US AA2003076606). By the technologies disclosed in these documents, off-axial optical systems free from chromatic aberration including five or six reflecting surfaces in which aberrations are satisfactorily corrected have been achieved in image reading apparatuses.
In the technology disclosed in US AA2003038228, since downsizing of the optical system is also aimed at, the embodiments disclosed therein are optical systems that are suitable for a carriage-integrated system. An embodiment of an imaging optical system used for image reading disclosed in US AA2003076606 is directed to an off-axial optical system including three reflecting surfaces, which has an optical path length long enough to apply it to a 2:1 mirror scanning type scanner.
In reflecting off-axial optical systems, it is difficult to maintain excellent optical performance while constructing all the reflecting surfaces as spherical surfaces. Accordingly, at least one rotationally asymmetrical aspherical surface (or a free curved surface) is introduced to achieve excellent optical performance.
It has been known that in optical systems composed of reflecting surfaces, decenter errors generally cause significant deterioration in optical performance. When a reflecting optical element having a rotationally asymmetrical aspherical surface (free curved surface) is introduced into an off-axial optical system, an extremely high degree of accuracy in the position of a member for holding that element is required, and the degree of accuracy required is higher than in the case of a normal spherical reflecting surface.
When, particularly, the number of surfaces is more than two, it is necessary to achieve accuracy in the relative position of the surfaces. A holding member having a complicated structure and a large size is required in order to achieve a high degree of accuracy. This leads to problems such as a decrease in the ease of assembly and an increase in the difficulty of manufacturing.
If an off-axial reflecting surface in the form of a free curved reflecting surface is to be produced using an ordinary glass material, a complicated production process will be required.
Such a surface may be produced using a plastic such as a polycarbonate, an acrylic or a polyolefin. However, in cases where the number of the surfaces is large, the problem of an increase in the manufacturing cost due to the cost for the corresponding number of molds will arise.
An object of the present invention is to provide an imaging optical system that does not suffer from asymmetrical aberration significantly even though it is composed of off-axial reflecting surfaces and can read images with a very simple structure while maintaining excellent image performance, and to provide an image reading apparatus using such an imaging optical system.
Particularly, the present invention is intended to provide an imaging optical system that is preferably used in constructing an image reading system in a digital copying machine, an image scanner or the like easily.